Rotary joints are used to transmit data from a rotating object, such as a shaft for example, to a stationary object, such as a closed-loop control module located in a switchgear cabinet.
In the case of standard commercially available rotary joints the data is transmitted by means of slip rings for example.
Also known are rotary joints in which data is transmitted from the rotating to the stationary part by means of optical signal transmission. In this case a transmitter disposed in the axial center of rotation of the shaft transmits a light signal to a stationary receiver that is likewise disposed in an axially central manner. However said type of optical rotary joint embodied in a standard commercially available configuration is unsuitable for transmitting signals if the shaft is embodied as a hollow shaft, since the transmitter cannot be disposed in the axial center of rotation of the shaft but must be disposed on the hollow shaft outside the center of rotation, with the result that data can only be transmitted when the shaft and therefore the transmitter are in a specific angular position such that transmission to the receiver, which is likewise disposed outside the center of rotation, is possible. When a single transmitter and a single receiver are used it is then generally only possible to transmit data just once per rotation of the shaft, at a specific angular position of the shaft, i.e. precisely when the transmitter and receiver are axially opposite each other, so that the light beam emitted by the transmitter can be received by the receiver.